A conventional optical disk apparatus records/reproduces data to/from an optical disk while performing a tracking operation based on a guide groove that is formed in advance on the surface of the optical disk so as to be winding in the radial direction, i.e., a wobble. FIG. 6 shows a configuration of a CD-R recording/reproduction apparatus as an example of a conventional optical disk apparatus. An optical pickup 100 irradiates an optical disk 101 being spun at a certain speed with laser light, and the reflected light is detected by a divided photodetector and converted into an electric signal. For the obtained signal, a servo signal producing circuit 102 produces a focus and tracking error signal to perform a focus and tracking servo control of the optical pickup 100 through a servo controller 103.
FIG. 7 is a diagram showing on an enlarged scale the surface of the optical disk 101 under a servo control. Under a servo control, laser light output from the optical pickup 100 is condensed into a spot as shown by a laser spot 110, and the spot is controlled by a tracking control so as to be always along a guide groove 111. If information is recorded on the optical disk 101, there are pits 112 along the guide groove 111, and the optical pickup 100 produces a signal based on the reflected light whose intensity changes depending on the presence/absence of the pits 112.
Referring back to FIG. 6, a wobble signal extraction circuit 104 extracts a wobble signal caused by the guide groove 111 shown in FIG. 7, i.e., a wobble. The servo controller 103 performs a spindle servo control by controlling the rotational speed of a spindle motor 105 so that the frequency of the wobble signal becomes equal to an intended value. Absolute time information (ATIP: Absolute Time In Pre-groove) that indicates address information of the optical disk 101 is embedded in the wobble signal. An ATIP decoder 106 decodes ATIP from the wobble signal, and sends, to a CPU 107, position information of the optical pickup 100 in the optical disk 101. A data signal producing circuit 108 produces a data signal from the signal produced by the optical pickup 100. A decoder 109 decodes the data signal produced by the data signal producing circuit 108 to decode the signal recorded on the optical disk 101.
The wobble signal extraction circuit 104 of the above optical disk apparatus will now be described in detail. FIG. 8 shows a configuration of the conventional wobble signal extraction circuit 104. The wobble signal extraction circuit 104 receives, from the optical pickup 100, optical disk signals S1 and S2 containing an RF signal component being a data reproduction signal and a wobble signal component caused by a wobble formed on the optical disk 101, and subtracts, at a subtracter 120, the optical disk signal S2 from the optical disk signal S1 to extract a wobble signal.
FIG. 9 show diagrams for illustrating the principle of the wobble signal extraction operation performed by the wobble signal extraction circuit 104. When reproducing a recorded optical disk, light 130 is ideally reflected off the disk surface equally by portions A, B, C and D of a photodetector 131, as shown in FIG. 9(a). Then, an A+D signal produced from the portions A and D arranged next to each other in the disk rotation direction (i.e., the optical disk signal S1 shown in FIG. 8) and a B+C signal produced from the portions B and C similarly arranged next to each other in the disk rotation direction (i.e., the optical disk signal S2 shown in FIG. 8) have RF signal components of the same phase and of the same amplitude, and have wobble signal components of reversed phases and of the same amplitude. Therefore, by subtracting the optical disk signal S2 from the optical disk signal S1 with the subtracter 120 shown in FIG. 8, the RF signal components contained in these signals are canceled out, whereby an (A+D)−(B+C) signal that contains only the wobble signal component can be extracted as the output signal of the subtracter 120.
However, due to factors such as the limit on the precision in the placement of the photodetector 131 when manufacturing an optical disk apparatus, light 130′ is reflected asymmetrically with respect to the photodetector 131 as shown in FIG. 9(b). Then, the RF signal components and the wobble signal components contained in the A+D signal and the B+C signal will be of different amplitudes. In view of this, the optical disk signals S1 and S2 are passed to the subtracter 120 after being adjusted through automatic gain controllers (hereinafter referred to as “AGC circuits”) 121 and 122 so that they will have the same amplitude. Moreover, an RF signal component remaining in the output from the subtracter 120 is removed by a bandpass filter 123, thus extracting a wobble signal S3. Then, the extracted wobble signal S3 is digitized by a digitizer circuit 124, and is used as a clock in the optical disk apparatus for a spindle servo control and for obtaining disk address information (see, for example, Patent Document 1).
Optical disks currently available in the market include WORM-type optical disks including DVD-R in addition to CD-R, and rewritable optical disks such as CD-RW, DVD-RAM and DVD-RW. A recording/reproduction apparatus compatible with these optical disks also extracts a wobble signal from an optical disk in a manner similar to that described above to produce a necessary clock signal when recording data.
(Patent Document 1) Japanese Laid-Open Patent Publication No. 8-194969